“What do you think, Susı̄ma, is form permanent or impermanent?” –“Impermanent, venerable sir.”–“Is what is impermanent suffering or happiness?”–“Suffering, venerable sir.”–“Is what is impermanent, suffering, and subject to change fit to be regarded thus: ‘This is mine, this I am, this is my self’?”–“No, venerable sir.”

For any commentator (either a revered near-contemporary of the Buddha, BB, or someone here on DW) to know that Susima had achieved "insight knowledge" or "knowledge of the stability of the Dhamma", then either they are aware of some unexplicated textual detail, or they know something of Susima from another source.

If we held a poll here on DW with the above questions, many of us would give the same answers as Susima, wouldn't we? I can't see from the text alone that Susima "knows" any more than I would appear to know, notwithstanding any commentarial traditions to the contrary...

Most of us are doing no more than running through the alternatives to "truly personal knowledge". And as you rightly point out, the third-person account of Susima's liberation or breakthrough is lacking here. And to this we can add our knowledge that he is at the outset a thief of the Dhamma...

Then the Venerable Susı̄ma prostrated himself with his head at the Blessed One’s feet and said: “Venerable sir, I have committed a transgression in that I was so foolish, so confused, so inept that I went forth as a thief of the Dhamma in such a well-expounded Dhamma and Discipline as this. Venerable sir, may the Blessed One pardon me for my transgression seen as a transgression for the sake of future restraint.”

“Surely, Susı̄ma, you have committed a transgression in that you were so foolish, so confused, so inept that you went forth as a thief of the Dhamma in such a well-expounded Dhamma and Discipline as this. ...

“Surely, bhikkhus, you have committed a transgression—so foolish, so confused, so inept were you—in that, having gone forth in such a well-expounded Dhamma and Discipline, you competed with each other in regard to your learning…. But since you see your transgression as a transgression and make amends for it in accordance with the Dhamma, we pardon you for it. For it is growth in the Noble One’s Discipline when one sees one’s transgression as a transgression, makes amends for it in accordance with the Dhamma, and undertakes future restraint.”